Modern vehicle navigation systems may generate and display real-time positioning and direction information to assist drivers. Such systems can also provide critical location information for emergency service providers. Many modern vehicle navigation systems may use a combination of positioning systems in order to improve accuracy and robustness. For example, such systems may use sensor-based Dead Reckoning (DR) to bridge Global Positioning System (GPS) gaps. Dead Reckoning navigation systems may be based upon two general categories of sensors. The first category may include inertial motion sensors (IMUs), such as, for example, accelerometers and gyroscopes. Sensors in this category are not only useful in vehicle navigation systems, but may also be used in other vehicle sub-systems, such as stability control systems. The second category of sensors may include Wheel Speed Sensors (WSS) (also known as wheel tick sensors) and vehicle odometers. These sensors may be included in anti-lock brake systems for detecting wheel slippage and skidding. Both categories of sensors may produce data that can be accessed by the vehicle navigation system through a wired and/or wireless connection to the sensors themselves, or to the vehicle data bus containing sensor data.
Errors associated with DR position systems which may be difficult to mitigate are dynamic errors caused by the vehicle maneuvers and/or the vehicle riding on a non-horizontal surface. In these cases, due to the compound effect of the vehicle's suspension and tire elasticity, the vehicle body can rotate relative to the road surface causing inertial sensor misalignment and consequently the misinterpretation of their data. Tire elasticity by itself may cause yet another DR error because the wheel speed sensor and vehicle odometer signal's incorporation in the DR system typically needs accurate knowledge of each tire radius. Unknown changes in tire radii may result in errors in the computed travel distance and turn angle. In another vehicle application related to safety, WSS data may also be utilized for flat tire detection. When the tire radius, which may be calibrated using GPS, suddenly shrinks considerably, it may be an indication of a flat tire. As with DR positioning, the tire radii should be accurately known for a reliable results.
Depending on the road surface orientation (i.e., road profile) and vehicle maneuvers, the vehicle suspension may either roll to the side, or tilt nose up/down, or do both. This motion may typically be accompanied by the body rotation due to tire size change, so the total vehicle body orientation change will be the sum of both rotations due to suspension shift and tire deformation. As it was noted above, the tire radii change itself may be important for the wheel tick based DR and flat tire detection. Specific examples of these effects are illustrated in FIGS. 1-4 and described in more detail below.
FIG. 1 illustrates an example 100 of the vehicle dynamics associated with a left turn on traveling on a flat road surface. The rear of the vehicle 105 is illustrated, and shows the vehicle roll and tire deformation that occurs during this maneuver. During the left turn, the torque of the “inertial” centrifugal force about the center of mass 110 (vehicle mass times centripetal acceleration shown in FIG. 1 as −m{right arrow over (a)}centr) causes the vehicle to roll to the right (passenger) side. This force causes the right tire 130 to compress relative to the left tire 125, and the right side of the suspension 120 to compress relative the left side of the suspension 115.
FIG. 2 illustrates an example 200 of the dynamics associated with the vehicle 105 traveling on a banked road surface. Again, the rear of the vehicle 105 is illustrated, and shows the vehicle tilting to the right side as it travels straight upon the banked road. The torque (m{right arrow over (g)}), caused by gravitational acceleration, will cause the vehicle to tilt, thus resulting in the right tire 130 compressing relative to left tire 125, and the right side of the suspension 120 compressing relative to the left side of the suspension 115. Accordingly, in both examples 100 and 200, the right wheel 130 radius will shrink and the left wheel 125 radius will increase, and the suspension will skew clockwise.
FIG. 3 illustrates an example 300 of the dynamics associated with the vehicle 105 accelerating forward on a flat road surface. The passenger side of the vehicle 105 is illustrated, and shows the vehicle pitch and tire deformation that occurs during this maneuver. Specifically, the forward acceleration causes the vehicle 105 to rotate nose up, increasing the load on the rear tires 330 to compensate for torque of the “inertial” force (vehicle mass times forward acceleration shown as −m{right arrow over (a)} in FIG. 3). The inertial force (−m{right arrow over (a)}) will cause the rear suspension 315 to shrink and the front suspension 310 to expand, thus resulting in a skewed suspension creating a positive pitch rotation. This force will also cause the radii of both rear wheels 330 to shrink and the radii both front wheels 305 to expand, thus contributing the vehicle pitch rotation.
FIG. 4 illustrates an example 400 of the dynamics associated with the vehicle 105 traveling uphill along an inclined road surface. The inclination cause the vehicle to rotate about its center of mass 110, thus resulting causing a vehicle pitch and tire deformation similar to that described above in FIG. 3. Here, the force (−m{right arrow over (g)}) caused by gravitational acceleration results in rear suspension 315 shrinking and the front suspension 310 to expanding, thus again creating a positive pitch rotation. This force will also cause the radii of both rear wheels 330 to shrink and the radii both front wheels 305 to expand, thus again contributing the vehicle pitch rotation.
As can be seen from the above described examples, different types of vehicle maneuvers can result in various dynamic errors that can affect the accuracy of both WSS and IMU sensors. Because these errors can adversely affect the accuracy of DR positioning, it would be desirable to calibrate the data provided such sensors in order to improve the accuracy of the vehicle navigation system.